1. Field of the Invention
This invention relates to performing radiated shielding effectiveness measurements and in particular to a means and method of performing radiated shielding effectiveness measurements using mode-stirred chambers.
2. Description of the Prior Art
A mode-stirred chamber is a modified shielded room that can range in size from small metal enclosures, e.g., a 2 cubic meter box, to large shielded rooms. To qualify for use as a mode-stirred chamber an enclosure must be a conductive, radio frequency(rf)-tight enclosure. The enclosure's smallest dimension must be large compared to the operating wavelength. This size-frequency relationship imposes a lower frequency limitation which generally restricts operations to frequencies higher than 200 MHz for most shielded rooms.
The unique feature of a mode-stirred chamber is the "paddle wheel" tuner, a relatively large field-perturbing device shaped like a paddle wheel that is rotated within the chamber. This irregularly shaped metal structure causes very large changes in the standing-wave patterns within the chamber as it rotates. In this way, the many simultaneously existing modes are "stirred" just as in the "Evaluation and Use of a Reverberation Chamber for Performing Electromagnetic Susceptibility Vulnerability Measurements" by M. L. Crawford and G. H. Koepke in the National Bureau of Standards Technical Note 1092, April 1986.
Shielding effectiveness methods using the mode-stirred chamber have proven to be easy to perform and repeatable. The traditional approach, referred to as the discrete frequency method, involves testing one frequency at a time. Energy is concentrated at discrete frequencies thereby yielding extremely accurate data. However, testing time for each frequency may take as long as forty minutes per frequency. If for example, the frequencies to be tested ranged from 2 to 4 GHz and measurements were made every 250 MHz, the total time for the test would be 360 minutes. Moreover, only nine discrete frequencies would be tested over the desired operating range.